Back in the early 80's, somebody in the family built a chicken coop from outdoor plywood treated with rolled-on used motor oil, which made a rich dark brown. It is still holding up in tropical Florida, so how's that for endurance?

Woodworking sites would be a good source for advice on this. Plywood can be blotchy when stained, especially depending on the exterior wood. Still, the simplest approach would be either a dark stain (walnut perhaps) with a polyurethane finish (the stuff that's used for hardwood floors would be highly durable).

Durtex is great for portable use. Extremely durable and tintable. Looks like bedliner however. You could also cover the plywood with Tolex (sort of like a sheet of vinyl) Hundreds of colors and surface textures available.

For home use, you could veneer the plywood. Veneers will give you a furniture quality look. If it is nice plywood like Baltic Birch (the preferred speaker plywood), and your construction techniques are cosmetically neat, you could dye it first, then shellac or polyurethane it. Stains have ground pigmemts in them that clog the woods pores, that's why some woods are difficult to get to take the stain and can look blotchy. Dyes OTOH, are homogenous and create a more even look. Go to a place like Woodcrafters, that's where the wood geeks hang out.

"Baltic" or similar grades of multi-ply - often with maple face veneers, will usually take a "natural" clear coat of shellac, lacquer, conversion varnish, Tung oil, etc quite nicely, but may get a bit blotchy with darker penetrating stains unless first sealed / treated with purpose made products.

The choice of preparation product should be compatible with choice of final finishing materials - for example:

Some woods will blotch naturally because of the uneven concentration of pores in the wood structure(pine,maple,birch,cherry etc)there are a couple of ways to combat this, wood conditioner will partially fill the pores letting the stain penetrate more evenly it will also lighten the colour of the stain, you can purchase or make your own by mixing 1 part boiled linseed to about 8 parts mineral spirits, this ratio can be adjusted slightly and does not need to be exact another solution is to apply a thin 1lb cut of shellac before staining let dry and lightly sand before staining.

Using solid wood All woods will move seasonlly with changes in humidity, some more than others and different cuts of the same species will move in varying amounts for instance quarter sawn is the stablest wood to use if movement is a issue, flat sawn will move more and can cup 99% of the time in the opposite direction of the of the circular curve of the growth rings, surprisingly quarter sawn eastern white pine has one of the least amount of movement in service. I have actually used laminated strips of qs pine as a sub-strait and hand cut veneers applied to both sides, a 8" wide piece of this laminate has been sitting in my basement shop for several years now and it is still structurally sound. Lee valley sells a very useful wood movement reference guide with about 80 different species listed, a simple math calculation allows you to see how much your wood will move seasonally
PS any solid wood you want to use in your projects should be left in your shop for a period of time(at least a week) in a way that air can circulate around it and the wood can acclimate and bring all the moisture content together.
Sorry if i placed this in the wrong thread